1. Field of the Invention
This invention relates to electrical power generation.
2. Description of Related Art
Electrical power for military and commercial aircraft is typically generated by either an AC or DC generator that is controlled by regulating the voltage at a xe2x80x9cpoint-of-regulationxe2x80x9d (POR) to a specified level. To regulate the POR voltage, a generator control unit (GCU) modulates a generator excitation source voltage so that an ideal excitation current is maintained according to the load and speed conditions. When load on the generator is increased, the GCU must increase an exciter field current to compensate for a POR voltage drop caused by the extra load. When load on the generator decreases, the GCU must reduce the generator exciter field current so that the POR voltage will not be too high. In other words, the GCU must compensate for load transitions (e.g., high load to low load, or vice versa) by increasing or decreasing the exciter field current. If load transition compensation is not achieved quickly, the POR voltage could fall outside a specified limit, thereby causing utilization equipment malfunction and/or damage.
FIG. 1 illustrates a conventional generator control configuration for typical aircraft AC power generation systems. As seen in FIG. 1, a conventional generator control unit 10 includes the following main elements: (a) a field current modulation switch 12; (b) a field current modulation switch driver 14; and (c) and a free wheeling diode 16. The generator control unit 10 is connected to an exciter stator winding 22 of a generator 20 via lines 17 and 18 to provide a field current If. As is well known, the flow of field current If through the exciter stator winding 22 of an AC generator induces a voltage in an exciter rotor winding 24 of the generator 20, which is rectified by a rectifier 26. The resulting rectified voltage is applied to a field winding 28, which induces an AC voltage in a generator main winding 29 for distribution to electrical loads of the aircraft via feeders (not shown). The field current modulation switch 12, which is typically either a MOSFET (metal-oxide-semiconductor field-effect transistor) or an IGBT (insulated-gate field-effect transistor), is connected to line 18 and between the exciter stator winding 22 of the generator 20 and ground. The field current modulation switch 12 is repeatedly switched between an ON state and an OFF state by the field current modulation switch driver 14, such that the duty cycle (or ON/OFF pulse width) of the field current modulation switch 12 maintains the field current flowing trough the exciter stator winding 22 at a given level as a function of load.
During normal operation, the field current If should be kept continuous to maintain a ripple-free POR voltage and to reduce the voltage noise across the field current modulation switch 12 during its switching. The free wheeling diode 16 is connected between lines 17 and 18, the anode being connected to line 18 between an output end of the exciter stator winding 22 and the field current modulation switch 12 and the cathode being connected to line 17 between an input end of the exciter stator winding 22 and a DC power source (not shown), and bypasses excitation energy stored in the exciter stator winding 22 when the field current modulation switch 12 is OFF. When the field current modulation switch 12 is ON, the diode 16 is reverse-biased and is in a blocking state. Therefore, excitation current on line 18 from the exciter stator winding 22 goes to ground through the field current modulation switch 12. When the field current modulation switch 12 is in the OFF state, the diode 16 is forced on by the induced voltage of the exciter stator winding 22 and the energy in the winding keeps flowing through the diode 16 via the line 18 so that the current flowing though the diode 16 is included in the field current If that is fed to the input end of the exciter stator winding 22 via line 17, thereby achieving a smooth continuous field current If. Thus, the diode 16 creates a free-wheeling path for excitation energy from the exciter stator winding 22.
For aircraft with traditional fixed frequency electrical systems that normally operate around 400 Hz, the conventional configuration in FIG. 1 does not pose a serious performance problem. The inventors of this application have found, however, that problems may arise in variable frequency systems that have gained more attention in recent years. For a typical aircraft, electric power generated from a generator has to meet power characteristic requirements dictated by either the industry or military standards. One typical requirement is the maximum voltage overshoot and recovery time during a step load removal transient elsewhere in the electric power utilization system. In a variable frequency generator control system, in which the generator speed range can be wide from 10000 to 24000 revolutions-per-minute, if the energy stored in the exciter winding cannot be depleted quickly enough when the field current modulation switch is OFF, the voltage overshoot often exceeds acceptable levels when the load is taken off from the generator at high speed, especially when a large generator is used. Furthermore, during generator power-up at high generator speeds, the generator voltage can be excessive due to inability of the generator control unit to accurately ramp-up the excitation current because energy stored in the exciter stator winding does not decay fast enough. Because there is not much that can be done to solve these problems with existing conventional control compensation in standard generator control units, a product cannot be delivered to a customer if it fails to meet the customer""s voltage overshoot tolerances.
In accordance with the present invention, the above drawbacks of the conventional power generator control configuration are resolved through use of a generator control unit that selectively and temporarily introduces an energy absorbing circuit into an excitation current free-wheeling path to absorb residual energy from a generator winding, thereby accelerating the field current decay rate to reduce voltage overshoot in the generator. According to the present invention, the energy absorbing circuit is selectively and temporarily introduced into the excitation current free-wheeling path when the generator experiences a load transition (e.g., a transition from high load to low load) and/or during a power-up stage to reduce voltage overshoot.
According to an embodiment of the present invention, a power generator control unit includes a field current modulator that is repeatedly switched between an ON state and an OFF state to control the flow of field current to the an exciter winding of the power generator, and excitation current from the exciter winding is fed back to the generator via a free-wheeling diode when the field current modulator is OFF. An energy absorbing circuit is selectively added to the free-wheeling path. A by-pass switch is provided across the energy absorbing circuit, which my be an RC circuit, to provide, when in an ON state, a low-impedance path for excitation current to flow in the free-wheeling path. In this implementation, the generator control unit includes an impedance by-pass driver that changes the by-pass switch from an ON state to an OFF state as a function of one or more detection signals, e.g., indicating a load transition or power-up condition, to selectively and temporarily introduce the energy absorbing circuit into the free-wheeling path to accelerate decay of the excitation current from an exciter winding of the generator.